The invention relates to a biaxially stretched polypropylene monofilm made from pure polypropylene and having an ash content of less than 50 ppm. Polypropylene monofilms of this type have surfaces of the same or comparable roughness.
Polypropylene films for use as electrical insulation films must have a multiplicity of electrical, mechanical, and surface properties. Examples of desired electrical properties include a low dielectric loss factor tan .delta.), high electrical volume resistance, high d.c. and a.c. voltage strength, and the lowest possible number of flaws.
Furthermore, electrical insulation films are required to have high strength in the longitudinal direction and low shrinkage. In addition, in films, in particular for use in metallization, for example for metallized polypropylene film dielectric capacitors, there is a requirement for a smooth surface, which is metallized, and a rough surface, which is necessary for processing reasons in order to avoid blocking of the film coil. The requirement for a smooth side of the film results from the desire to achieve the most flaw-free metallization possible, which ensures a long service life of a capacitor produced from the metallized film.
This essential product property of capacitors made from metallized polypropylene films, in particular in the case of metallization using aluminum, is determined by measuring the drop in capacity over a time span of from about 1000 to 2000 hours under defined load, level of the applied alternating voltage, and frequency. A long service life means a low drop in capacity.
The drop in capacity results from oxidation processes on the vapor-deposited metal layer. These oxidation processes cause nonconductive aluminum oxide, Al.sub.2 O.sub.3, to form in the case of films vapordeposited with aluminum. These oxidized, nonconductive areas of the aluminum layer grow with time and reduce the active capacitor surface area and thus the capacity of the capacitor. The mechanisms responsible for the corrosion are of an electrochemical nature. Thus, "On the mechanism of aluminum corrosion in metallized film AC capacitors", IEEE Transactions on Electrical Insulation, Volume E1-19, No. 4, August 1984, author D.F. Taylor, describes that complete removal of moisture causes cessation of corrosion, but such removal is impossible in practice. On the other hand, corrosion increases with temperature, applied alternating voltage, and frequency. A directcurrent field, which is equivalent to the peak value of the alternating-current field, can neither initiate nor support the corrosion process.
These observations are consistent with the mechanism of anodic oxidation at the edge of flaws in aluminum coatings of metallized electrical insulation films. These discontinuities in the aluminum layer must be sufficiently large to reinforce the local electrical field and to maintain ion transport into the oxide site. Fibrils and pits on the polymer surface and both electrical and electrochemical self-healing processes are the most probable sources of corrosion which initiate defects in the metal coating. As is known, the drop in capacity increases with the roughness of the surface of the side of the film to be metallized.
A further known problem in the further processing of polypropylene films having very smooth surfaces is that the films tend to block during cutting and winding. In particular during further processing on high-speed capacitor winding machines, such films with smooth surfaces on both sides cannot be employed. For this reason, polypropylene electrical insulation films are required to have adequate roughness on one side, which ensures that sufficient air is present between two successive layers of a film coil to prevent blocking of the film and thus to ensure good processing properties.
DE-A 28 51 557 discloses a biaxially stretched polypropylene composite film which comprises two layers of different composition, one surface having a maximum roughness R.sub.Z of 0.15 .mu.m, while the other surface of the film composite has a R.sub.Z value of between 0.2 .mu.m and 0.4 .mu.m. The modulus of elasticity of this composite film in all directions of the film plane is greater than 2000 N/mm.sup.2. The film composite has a d.c. voltage strength of 690 V/.mu.m.
A film composite of this type is used as an electrical insulation film for the production of metallized polypropylene film dielectric capacitors, a certain roughness being absolutely necessary on one side for processing reasons, while the other side of the film composite is smooth. In order to achieve the different roughness, the layer on the smooth side of the film composite comprises pure polypropylene and the other, rough side, comprises a polymer mixture or a block copolymer. A film composite of this type exhibits a low drop in capacity. In addition to increased costs for raw materials and production, due to the melt coating or coextrusion of the two layers, the technological problem arises that partial metal delamination can occur on further processing of the film composite in the case of this known film composite.
DE-A 27 40 237 describes a production process for a polypropylene film having rough surfaces in which the roughness is produced by setting certain crystallization temperatures and cooling periods as desired. DE-A 25 53 693 discloses a process for the production of rough polypropylene films in which a fibril-like network structure is achieved on the surface by means of certain stretching temperatures and ratios. This network structure ensures better absorption of the insulation oils used in the production of capacitors.
A similar process for the production of rough electrical insulation films is disclosed in DE-A 27 22 087. Here, specific stretching conditions and temperatures cause crystals in the .beta.-form, having a mean diameter of more than 7 .mu.m, to be produced in the surface, with a distribution density of the crystals of greater than 5000 g/cm.sup.2 in the case of spherulites or greater than 1000 g/cm.sup.2 in the case of rod-like crystals. DE-A 26 01 810 describes a certain temperature program for roughening tubular films, giving spherulites of the .gamma.-type.
Due to the steep characteristic line of the crystallization kinetics, the known processes have the problem that the observance of very specific process conditions is extremely difficult technically and is very susceptible to faults due to external influences, such as, for example, air convection. The resultant films are primarily designed for absorption of insulation oils employed in the production of capacitors, but less for the electrical properties as electrical insulation films.
It is known that the electrical properties of biaxially stretched polypropylene films are highly dependent on their surface structure. The ideal electrical properties are observed on the one hand in polypropylene films having very smooth and planeparallel surfaces, but on the other hand the problem outlined above of blocking of the film during further processing occurs. The good electrical properties of polypropylene films diminish with increasing roughness of the surface. In particular the service life test, in which the drop in capacity of a capacitor is measured as a function of time, indicates a clear dependence on the surface roughness.
Vapor-deposition of the polypropylene films with a rough surface gives a non-constant layer thickness since the metal layer is thinner at the edges of a fibril or bump than at the flat areas of the film.